A Simple Method to Optimally Select Upper-Limb Joint Angle Trajectories from Two Kinect Sensors during the Twisting Task for Posture Analysis.

A trunk-twisting posture is strongly associated with physical discomfort. Measurement of joint kinematics to assess physical exposure to injuries is important. However, using a single Kinect sensor to track the upper-limb joint angle trajectories during twisting tasks in the workplace is challenging due to sensor view occlusions. This study provides and validates a simple method to optimally select the upper-limb joint angle data from two Kinect sensors at different viewing angles during the twisting task, so the errors of trajectory estimation can be improved. Twelve healthy participants performed a rightward twisting task. The tracking errors of the upper-limb joint angle trajectories of two Kinect sensors during the twisting task were estimated based on concurrent data collected using a conventional motion tracking system. The error values were applied to generate the error trendlines of two Kinect sensors using third-order polynomial regressions. The intersections between two error trendlines were used to define the optimal data selection points for data integration. The finding indicates that integrating the outputs from two Kinect sensor datasets using the proposed method can be more robust than using a single sensor for upper-limb joint angle trajectory estimations during the twisting task.

DR-SIP: protocols for higher order structure modeling with distance restraints- and cyclic symmetry-imposed packing.

MOTIVATION: Quaternary structure determination for transmembrane/soluble proteins requires a reliable computational protocol that leverages observed distance restraints and/or cyclic symmetry (Cn symmetry) found in most homo-oligomeric transmembrane proteins. RESULTS: We survey 118 X-ray crystallographically solved structures of homo-oligomeric transmembrane proteins (HoTPs) and find that ∼97% are Cn symmetric. Given the prevalence of Cn symmetric HoTPs and the benefits of incorporating geometry restraints in aiding quaternary structure determination, we introduce two new filters, the distance-restraints (DR) and the Symmetry-Imposed Packing (SIP) filters. SIP relies on a new method that can rebuild the closest ideal Cn symmetric complex from docking poses containing a homo-dimer without prior knowledge of the number (n) of monomers. Using only the geometrical filter, SIP, near-native poses of 7 HoTPs in their monomeric states can be correctly identified in the top-10 for 71% of all cases, or 29% among 31 HoTP structures obtained through homology modeling, while ZDOCK alone returns 14 and 3%, respectively. When the n is given, the optional n-mer filter is applied with SIP and returns the near-native poses for 76% of the test set within the top-10, outperforming M-ZDOCK's 55% and Sam's 47%. While applying only SIP to three HoTPs that comes with distance restraints, we found the near-native poses were ranked 1st, 1st and 10th among 54 000 possible decoys. The results are further improved to 1st, 1st and 3rd when both DR and SIP filters are used. By applying only DR, a soluble system with distance restraints is recovered at the 1st-ranked pose. AVAILABILITY AND IMPLEMENTATION: https://github.com/capslockwizard/drsip. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

Prospective gait changes as a function of shifting perceptions of slipperiness: effects of visual and somatosensory cues.

Forty participants, ages 18-45 years, rated perceived slipperiness before and after walking on five different floors under three different surface conditions. The before-ratings were taken as a proxy for visual cues to slipperiness, while after-ratings were taken as a proxy for somatosensory feedback received while walking on the surface. Before and after ratings of slipperiness were used to predict gait parameters, as a function of trial, during repeated walking. Effects of after-ratings of slipperiness were observed beginning on the second trial, and continued through the fifth trial, while effects of before-ratings of slipperiness were most apparent on the first trial. When perceived slipperiness increased (or decreased) from before to after walking on the surface, gait became more (or less) protective across trials. It is concluded that both visual cues, as well as somatosensory feedback, are used in the prospective control of gait. Practitioner Summary: Effects of visual and somatosensory cues to slipperiness on gait were disentangled using floor surfaces varying in the slipperiness suggested by those cues. Visually based ratings of slipperiness predicted gait parameters on earlier trials, while somatosensory-based ratings predicted gait parameters on subsequent trials. Flooring design should provide reliable information regarding slipperiness.

Straight ladder inclined angle in a field environment: the relationship among actual angle, method of set-up and knowledge.

Ladder inclined angle is a critical factor that could lead to a slip at the base of portable straight ladders, a major cause of falls from heights. Despite several methods established to help workers achieve the recommended 75.5° angle for ladder set-up, it remains unclear if these methods are used in practice. This study explored ladder set-up behaviours in a field environment. Professional installers of a company in the cable and other pay TV industry were observed for ladder set-up at their worksites. The results showed that the actual angles of 265 ladder set-ups by 67 participants averaged 67.3° with a standard deviation of 3.22°. Although all the participants had training on recommended ladder set-up methods, only 3 out of 67 participants applied these methods in their daily work and even they failed to achieve the desired 75.5° angle. Therefore, ladder set-up remains problematic in real-world situations. Practitioner Summary: Professional installers of a cable company were observed for portable straight ladder set-up at their worksites. The ladder inclined angle averaged 67.3° with a standard deviation of 3.22°, while the recommended angle is 75.5°. Only a few participants used the methods that they learned during training in their daily work.

Investigating gait adjustments and body sway while walking across wooden scaffold boards.

The flexible wooden boards are still being used on the scaffolds at some construction worksites in China. We examined if the board dimension, acclimation phase and construction work experience influence workers' walking strategies on these boards. Among the 40 construction workers recruited, half of them had more than 1 year of construction work experience. The participants' body sway and the gait adjustments were captured and analysed. Our analysis showed the width and thickness of the boards had significant effects on walking speed. There is a significant interaction between construction experience and board width on stride width, whereas the interaction between construction experience and acclimation phase affects step length and walking speed. The body sway at the C6/C7 and L5/S1 locations are significantly affected by most of the factors. We also found that once the participants acclimated to the board condition, the sway variability increased. Practitioner Summary: This study investigated gait and postural adjustments of workers with different construction experience while walking over wooden scaffold boards, which is one of the activities highly associated with falling accidents in China.

The natural angle between the hand and handle and the effect of handle orientation on wrist radial/ulnar deviation during maximal push exertions.

The purpose of this experiment was to quantify the natural angle between the hand and a handle, and to investigate three design factors: handle rotation, handle tilt and between-handle width on the natural angle as well as resultant wrist radial/ulnar deviation ('RUD') for pushing tasks. Photographs taken of the right upper limb of 31 participants (14 women and 17 men) performing maximal seated push exertions on different handles were analysed. Natural hand/handle angle and RUD were assessed. It was found that all of the three design factors significantly affected natural handle angle and wrist RUD, but participant gender did not. The natural angle between the hand and the cylindrical handle was 65 ± 7°. Wrist deviation was reduced for handles that were rotated 0° (horizontal) and at the narrow width (31 cm). Handles that were tilted forward 15° reduced radial deviation consistently (12-13°) across handle conditions. PRACTITIONER SUMMARY: Manual materials handling (MMH) tasks involving pushing have been related to increased risk of musculoskeletal injury. This study shows that handle orientation influences hand and wrist posture during pushing, and suggests that the design of push handles on carts and other MMH aids can be improved by adjusting their orientation to fit the natural interface between the hand and handle.

The effect of handle design on upper extremity posture and muscle activity during a pouring task.

In this study, the effect of container handle parameters on shoulder and upper limb muscle activity and joint posture during a pouring task is investigated. Results indicated that a low handle position and a vertical handle slope minimised the loading of the shoulder muscles. A high and sloped handle minimised the muscle activity and wrist deviation of the lower arm. The effects of diameter were not significant for most dependent variables during the lifting phase of the task; however, beneficial effects were seen with the smallest handle diameter during the pouring phase. A trade-off existed between the shoulder and the hand/wrist posture with the different handles. The findings of significance with relatively small effect size suggest a high sensitivity of the system to any changes. In the real world, speed, space and work conditions are important factors that influence how a task is performed. This emphasises the importance of proper handle design. PRACTITIONER SUMMARY: In this study, the effect of container handle design on the muscle activity and postures of the upper extremity during a pouring task were analyzed using the experimental data collected from electromyography and motion tracking systems. The low handle height and vertical handle slope design yielded the lowest shoulder muscle activity.

Stochastic distribution of the required coefficient of friction for level walking--an in-depth study.

This study investigated the stochastic distribution of the required coefficient of friction (RCOF) which is a critical element for estimating slip probability. Fifty participants walked under four walking conditions. The results of the Kolmogorov-Smirnov two-sample test indicate that 76% of the RCOF data showed a difference in distribution between both feet for the same participant under each walking condition; the data from both feet were kept separate. The results of the Kolmogorov-Smirnov goodness-of-fit test indicate that most of the distribution of the RCOF appears to have a good match with the normal (85.5%), log-normal (84.5%) and Weibull distributions (81.5%). However, approximately 7.75% of the cases did not have a match with any of these distributions. It is reasonable to use the normal distribution for representation of the RCOF distribution due to its simplicity and familiarity, but each foot had a different distribution from the other foot in 76% of cases. PRACTITIONER SUMMARY: The stochastic distribution of the required coefficient of friction (RCOF) was investigated for use in a statistical model to improve the estimate of slip probability in risk assessment. The results indicate that 85.5% of the distribution of the RCOF appears to have a good match with the normal distribution.

Comparison of different methods to extract the required coefficient of friction for level walking.

The required coefficient of friction (RCOF) is an important predictor for slip incidents. Despite the wide use of the RCOF there is no standardised method for identifying the RCOF from ground reaction forces. This article presents a comparison of the outcomes from seven different methods, derived from those reported in the literature, for identifying the RCOF from the same data. While commonly used methods are based on a normal force threshold, percentage of stance phase or time from heel contact, a newly introduced hybrid method is based on a combination of normal force, time and direction of increase in coefficient of friction. Although no major differences were found with these methods in more than half the strikes, significant differences were found in a significant portion of strikes. Potential problems with some of these methods were identified and discussed and they appear to be overcome by the hybrid method. PRACTITIONER SUMMARY: No standard method exists for determining the required coefficient of friction (RCOF), an important predictor for slipping. In this study, RCOF values from a single data set, using various methods from the literature, differed considerably for a significant portion of strikes. A hybrid method may yield improved results.

Two linear regression models predicting cumulative dynamic L5/S1 joint moment during a range of lifting tasks based on static postures.

Previous studies have indicated that cumulative L5/S1 joint load is a potential risk factor for low back pain. The assessment of cumulative L5/S1 joint load during a field study is challenging due to the difficulty of continuously monitoring the dynamic joint load. This study proposes two regression models predicting cumulative dynamic L5/S1 joint moment based on the static L5/S1 joint moment of a lifting task at lift-off and set-down and the lift duration. Twelve men performed lifting tasks at varying lifting ranges and asymmetric angles in a laboratory environment. The cumulative L5/S1 joint moment was calculated from continuous dynamic L5/S1 moments as the reference for comparison. The static L5/S1 joint moments at lift-off and set-down were measured for the two regression models. The prediction error of the cumulative L5/S1 joint moment was 21 ± 14 Nm × s (12% of the measured cumulative L5/S1 joint moment) and 14 ± 9 Nm × s (8%) for the first and the second models, respectively. Practitioner Summary: The proposed regression models may provide a practical approach for predicting the cumulative dynamic L5/S1 joint loading of a lifting task for field studies since it requires only the lifting duration and the static moments at the lift-off and/or set-down instants of the lift.

Artificial neural network can improve the accuracy of a markerless skeletal model in L5/S1 position estimation during symmetric lifting.

This study investigated whether using an artificial neural network (ANN) method for L5/S1 position estimation based on the Kinect markerless skeletal model can produce more accurate data than measurements using the original Kinect skeletal model during symmetric lifting tasks. Twenty participants performed three symmetric lifting tasks twice at three vertical lifting height paths. Their postural data were simultaneously collected by a Kinect and a reference motion tracking system (MTS). The Kinect-based data are used as the model inputs, while its outputs are based on MTS. Three-layer ANN models to predict the L5/S1 position over the entire lifting duration were trained by identifying the relationship between the seven inputs (the participant's height and weight and the Kinect-based trunk angle, left knee angle, and left hip joint coordinates on the X-axis, Y-axis, and Z-axis) and three outputs (the reference L5/S1 position on the X-axis, Y-axis, and Z-axis). As a measure of error, the distances between the reference anatomical L5/S1 position and the predicted positions (by the ANN-Kinect system and the Kinect system) were calculated and compared. The results showed that introducing the ANN method can significantly (p < 0.0001) reduce the L5/S1 position estimation error (5.12 ± 1.83 cm) in comparison with directly using the original data output from the skeletal model driven by Kinect data (20.54 ± 3.24 cm). This method provides an alternative for L5/S1 position estimation while retaining the advantages of using Kinect such as portability, easy of use, and being equipped with the function of automatic skeletal identification.

Simple benchmarking method for determining the accuracy of depth cameras in body landmark location estimation: Static upright posture as a measurement example.

To evaluate the postures in ergonomics applications, studies have proposed the use of low-cost, marker-less, and portable depth camera-based motion tracking systems (DCMTSs) as a potential alternative to conventional marker-based motion tracking systems (MMTSs). However, a simple but systematic method for examining the estimation errors of various DCMTSs is lacking. This paper proposes a benchmarking method for assessing the estimation accuracy of depth cameras for full-body landmark location estimation. A novel alignment board was fabricated to align the coordinate systems of the DCMTSs and MMTSs. The data from an MMTS were used as a reference to quantify the error of using a DCMTS to identify target locations in a 3-D space. To demonstrate the proposed method, the full-body landmark location tracking errors were evaluated for a static upright posture using two different DCMTSs. For each landmark, we compared each DCMTS (Kinect system and RealSense system) with an MMTS by calculating the Euclidean distances between symmetrical landmarks. The evaluation trials were performed twice. The agreement between the tracking errors of the two evaluation trials was assessed using intraclass correlation coefficient (ICC). The results indicate that the proposed method can effectively assess the tracking performance of DCMTSs. The average errors (standard deviation) for the Kinect system and RealSense system were 2.80 (1.03) cm and 5.14 (1.49) cm, respectively. The highest average error values were observed in the depth orientation for both DCMTSs. The proposed method achieved high reliability with ICCs of 0.97 and 0.92 for the Kinect system and RealSense system, respectively.

Prediction accuracy in estimating joint angle trajectories using a video posture coding method for sagittal lifting tasks.

This study investigated prediction accuracy of a video posture coding method for lifting joint trajectory estimation. From three filming angles, the coder selected four key snapshots, identified joint angles and then a prediction program estimated the joint trajectories over the course of a lift. Results revealed a limited range of differences of joint angles (elbow, shoulder, hip, knee, ankle) between the manual coding method and the electromagnetic motion tracking system approach. Lifting range significantly affected estimate accuracy for all joints and camcorder filming angle had a significant effect on all joints but the hip. Joint trajectory predictions were more accurate for knuckle-to-shoulder lifts than for floor-to-shoulder or floor-to-knuckle lifts with average root mean square errors (RMSE) of 8.65 degrees , 11.15 degrees and 11.93 degrees , respectively. Accuracy was also greater for the filming angles orthogonal to the participant's sagittal plane (RMSE = 9.97 degrees ) as compared to filming angles of 45 degrees (RMSE = 11.01 degrees ) or 135 degrees (10.71 degrees ). The effects of lifting speed and loading conditions were minimal. To further increase prediction accuracy, improved prediction algorithms and/or better posture matching methods should be investigated. STATEMENT OF RELEVANCE: Observation and classification of postures are common steps in risk assessment of manual materials handling tasks. The ability to accurately predict lifting patterns through video coding can provide ergonomists with greater resolution in characterising or assessing the lifting tasks than evaluation based solely on sampling with a single lifting posture event.

The Gap Between Tools and Best Practice: An Analysis of Safety Prequalification Surveys in the Construction Industry.

This study characterizes safety prequalification surveys currently in use in the construction industry to identify approaches that include leading indicators of worker safety performance. We collected prequalification surveys available in the public domain from internet searches, construction company websites, published literature, and construction industry partners. We utilized a conceptual framework, based on safety theory and best practices, to categorize survey questions. Fifty-two prequalification surveys were identified containing 112 unique questions. Most included questions related to lagging indicators (83 percent), safety management leadership (75 percent), and worker training (60 percent). Safety management system elements such as hazard prevention and control, program evaluation and improvement, and coordination and communication were notably absent in 90 percent of the surveys. There was little consistency in the surveys available concerning leading indicators of safety. Only a small number of surveys currently in use incorporate all the elements of best practices associated with robust safety management systems.

Slipping of the foot on the floor when pulling a pallet truck.

Workers pulling pallet trucks are likely to slip when pulling and stepping on a low-friction floor. This study investigated the slipping of male participants when pulling a pallet truck, walking backward, and stepping on either a dry, wet, or glycerol-contaminated vinyl surface. The weight of the load on the truck was either low (0 kg), medium (295 kg), or high (568 kg). A motion-tracking system was used to collect the three-dimensional coordinates of the markers on the shoes. It was found that subjects might slip either upon landing of the leading foot on the toe (slip I) or before taking off of the lagging foot on the heel (slip II). The results indicated that the slip distances for both types of slip were significantly affected by the load and surface conditions and their interactions. Micro-slips (slips between 0.1 and 3 cm) and midi-slips (slips between 3 and 10 cm) were more common in slip I than in slip II. On glycerol-contaminated surfaces, the probabilities of a slide, or a slip more than 10 cm, for both slips I and II were over 40%. The implications of the results were discussed.

Rearward movement of the heel at heel strike.

This paper describes the observation of rearward movement (RM) of the heel following heel strike occurring during normal gait. Thirty-one participants recruited as part of a larger study on slip kinematics walked the length of an 8-m runway at a speed of 1.5 m/s. Several floor surfaces, presented dry and with contaminant, were used for the purpose of eliciting a wide range of small slip distances. The normal force applied to a forceplate mounted in the runway was used to identify heel strike, as well as to calculate the utilized coefficient of friction during early stance phase. A motion analysis system tracked the displacement of two heel-mounted markers, and the data were used to derive kinematic variables related to the heel strike event. Results showed that RMs occurred in 18.1% of 494 trials, with a mean rearward displacement of 5.02 (+/-3.68) mm. When present, RMs occurred in close temporal proximity to heel strike, typically completing RM within 40 ms of the heel strike event. When divided into groups by age, older participants (>40 years) were more than twice as likely to have RMs as younger participants. When grouped by height or weight, differences in the proportion of trials with RMs were small. In trials where RMs were observed, forward slip distances were significantly less than for trials with no RMs, 2.17 (+/-3.87) mm vs. 12.58 (+/-10.71) mm, respectively. The time until the heel stopped moving during the post-heel strike period was not significantly different between RM and non-RM trials. Further investigation of this gait feature may improve understanding of normal gait patterns and may have implications for future slipmeter development.

A methodology to quantify the stochastic distribution of friction coefficient required for level walking.

The required friction coefficient is defined as the minimum friction needed at the shoe and floor interface to support human locomotion. The available friction is the maximum friction coefficient that can be supported without a slip at the shoe and floor interface. A statistical model was recently introduced to estimate the probability of slip and fall incidents by comparing the available friction with the required friction, assuming that both the available and required friction coefficients have stochastic distributions. This paper presents a methodology to investigate the stochastic distributions of the required friction coefficient for level walking. In this experiment, a walkway with a layout of three force plates was specially designed in order to capture a large number of successful strikes without causing fatigue in participants. The required coefficient of friction data of one participant, who repeatedly walked on this walkway under four different walking conditions, is presented as an example of the readiness of the methodology examined in this paper. The results of the Kolmogorov-Smirnov goodness-of-fit test indicated that the required friction coefficient generated from each foot and walking condition by this participant appears to fit the normal, log-normal or Weibull distributions with few exceptions. Among these three distributions, the normal distribution appears to fit all the data generated with this participant. The average of successful strikes for each walk achieved with three force plates in this experiment was 2.49, ranging from 2.14 to 2.95 for each walking condition. The methodology and layout of the experimental apparatus presented in this paper are suitable for being applied to a full-scale study.

Relationship among several measurements of slipperiness obtained in a laboratory environment.

Multiple sensing mechanisms could be used in forming responses to avoid slips, but previous studies, correlating only two parameters, revealed a limited picture of this complex system. In this study, the participants walked as fast as possible without a slip under 15 conditions of different degrees of slipperiness. The relationships among various response parameters, including perceived slipperiness rating, utilized coefficient of friction (UCOF), slipmeter measurement and kinematic parameters, were evaluated. The results showed that the UCOF, perceived rating and heel angle had higher adjusted R2 values as dependent variables in the multiple linear regressions with the remaining variables in the final pool as independent variables. Although each variable in the final data pool could reflect some measurement of slipperiness, these three variables are more inclusive than others in representing the other variables and were bigger predictors of other variables, so they could be better candidates for measurements of slipperiness.

Association of subjective ratings of slipperiness to heel displacement following contact with the floor.

Falls precipitated by slipping are listed among the leading causes of occupational injuries. Several factors may influence the risk of slips and falls, including perception of surface conditions. The current research examined the relationship between perceptions of slipperiness and slip distance at heel strike. The investigation compared objective and subjective measures for 31 participants ranging in age from 19 to 67 years old. Subjective slipperiness ratings for several floor surface and walking velocity conditions were obtained. Small slips were not generally perceived, but a uniform slip distance threshold could not be identified due to the large variability in ratings. The Pearson correlation coefficient between slip distance and subjective ratings for slides (30-100mm) was r=-0.17. Results indicate that subjective ratings should be used cautiously as a measure of slipperiness, partially due to possible underestimation of slipperiness and the variability in perceptions.

The use of a heel-mounted accelerometer as an adjunct measure of slip distance.

A human-centered measure of floor slipperiness could be useful as an adjunct to conventional tribologic measures. This paper reports on the development and evaluation of a measure of slip distance based on variables derived from the signal of a heel-mounted accelerometer. Twenty-one participants walked on a laboratory runway under several surface slipperiness conditions at three walking speeds during a protocol designed to produce a wide range of slip distances at heel strike. Analysis of variance showed significant effects of slip distance (no-slip, micro-slip and slide), walking speed (1.52, 1.78 and 2.13 m/s) and their interactions on peak forward acceleration, peak vertical acceleration and deceleration time of the heel following heel strike in 704 trials. Regression analysis of slip distance and deceleration time showed the strongest relationship with R2=0.511. Large individual variation in the strength of this relationship was observed. The heel-mounted accelerometer may have utility as an adjunct measure in the evaluation of floor slipperiness, particularly for field applications where direct measurement may not be feasible.